Method for operating a process-measuring device

ABSTRACT

The process measuring device includes a flow sensor having a measuring tube, a sensor arrangement for producing a measurement signal, and an evaluation and operating circuit. The method serves to compensate for the effects of interfering potentials which are caused especially by foreign particles or air bubbles in the liquid to be measured. For this purpose, an anomaly in the waveform of the measurement signal caused at least in part by an electrical, especially pulse-shaped, interfering potential is detected by determining within a stored first data set a data group which digitally represents the anomaly. To generate an interference-free data set, the data belonging to the data group are removed from the stored first data set.

This application claims the benefit of Provisional Application No.60/485747, filed Jul. 10, 2003.

FIELD OF THE INVENTION

The invention relates to a method for operating a process measuringdevice, with which at least one physical, measured variable, especiallya flow rate, a viscosity, or the like, of a medium held in a processcontainer or flowing in a process pipeline is to be measured.Especially, the invention concerns a method for operating anelectromagnetic flowmeter, with which the volume flow rate of anelectrically conducting and flowing liquid is to be measured.

BACKGROUND OF THE INVENTION

In industrial process measurement technology, especially also inconnection with the automation of chemical or other industrialprocesses, so-called field devices, thus process measuring devicesinstalled near to the process, are employed for producing, on-site,measured-value signals as analog or digital representations of processvariables. Examples of such process measuring devices, known per se tothose skilled in the art, are described in detail in one or more of thefollowing references from the patent literature: EP-A 984 248, EP-A 1158 289, U.S. Pat. Nos. 3,878,725, 4,308,754, 4,468,971, 4,524,610,4,574,328, 4,594,584, 4,617,607, 4,716,770, 4,768,384, 4,850,213,5,052,230, 5,131,279, 5,231,884, 5,359,881, 5,363,341, 5,469,748,5,604,685, 5,687,100, 5,796,011, 6,006,609, 6,236,322, 6,352,000,6,397,683, WO-A 88 02 476, WO-A 88 02 853, WO-A 95 16 897, WO-A 00 36379, WO-A 00 14 485, WO-A 01 02816 and WO-A 02 086 426.

Examples of the process variables to be registered include a volume flowrate, a mass flow rate, a density, a viscosity, a fill or limit level, apressure or a temperature, or the like, of a process medium in the formof a liquid, powder, vapor, or gas conducted or available in acorresponding process container, such as e.g. a pipeline or a tank.

For registering the respective process variables, the process measuringdevice has a corresponding, usually physical-electrical, sensor, whichis placed in a wall of the container conducting the process medium or inthe course of a process pipeline conducting the process medium, andwhich serves for producing at least one measurement signal, especiallyan electrical signal, representing the primarily registered processvariable as accurately as possible. For this purpose, the sensor isadditionally connected with a suitable measuring device electronicsserving especially for a further processing or evaluation of the atleast one measurement signal. This includes usually an operating circuitdriving the sensor and a measuring and evaluation circuit for furtherprocessing of its measurement signals.

Process measurement devices of the described type are usually connectedtogether by way of a data transmission system connected to the measuringdevice electronics and/or with corresponding process control computers,to which they transmit the measured-value signals e.g. via a (4 mA to 20mA)-current loop and/or via digital data bus. Serving as datatransmission systems in such case are field bus systems, especiallyserial ones, such as e.g. PROFIBUS-PA, FOUNDATION FIELDBUS, with theircorresponding transmission protocols. The transmitted, measured-valuesignals can be processed further by means of the process controlcomputers and visualized as corresponding measurement results e.g. onmonitors and/or transformed into control signals for process adjustingactuators, such as e.g. magnetic valves, electromotors, etc.

For accommodating the measuring device electronics, such processmeasuring devices include, furthermore, an electronics housing, which,as e.g. proposed in U.S. Pat. No. 6,397,683 or WO-A 00 36 379, can besituated away from the process measuring device and connected therewithonly over a flexible cable, or which, as e.g. shown in EP-A 903 651 orEP-A 1 008 836, is arranged directly on the sensor or on a sensorhousing separately housing the sensor. Often, the electronics housingthen serves, as shown, for example in EP-A 984 248, U.S. Pat. No.4,594,584, U.S. Pat. No. 4,716,770 or U.S. Pat. No. 6,352,000, also foraccommodating some mechanical components of the sensor, such as e.g.membrane, rod, shell or tubular, deformation or vibration bodies, whichdeform during operation under the influence of mechanical loads; see, inthis connection, also the above-mentioned U.S. Pat. No. 6,352,000.

For measuring electrically conductive fluids, flowmeters having anelectromagnetic flow sensor are often used. In the following, ifexpedient, reference will be just to flow sensors, or flowmeters, forshort. As is known, electromagnetic flowmeters permit measurement of thevolume flow rate of an electrically conducting liquid flowing in apipeline and represent such measurement in the form of a corresponding,measured value; thus, per definition, the volume of liquid flowingthrough a pipe cross section per unit time is measured. Construction andmanner of operation of electromagnetic flowmeters are known per se tothose skilled in the art and are described in detail, for example, inDE-A 43 26 991, EP-A1 275 940, EP-A 12 73 892, EP-A 1 273 891, EP-A 814324, EP-A 770 855, EP-A 521 169, U.S. Pat. Nos. 6,031,740, 5,487,310,5,210,496, 4,410,926, 2002/0117009 or WO-A 01/90702.

Flow sensors of the described type usually each exhibit anon-ferromagnetic, measuring tube which is connected into the pipelinein a liquid-tight manner, for example by means of flanges or threadedjoints. The portion of the measuring tube which contacts the liquid isgenerally electrically non-conductive, so that no short circuit ispresent for a voltage induced in the liquid according to Faraday's lawof electromagnetic induction by a magnetic field cutting across themeasuring tube.

In keeping with this practice, metal measuring tubes are commonlyprovided internally with a nonconductive lining, e.g., a lining of hardrubber, polyfluoroethylene, etc., and are themselves generallynon-ferromagnetic; in the case of measuring tubes made entirely ofplastic or ceramic, particularly of alumina ceramic, the nonconductivelining is, in contrast, not necessary.

The magnetic field is produced by means of two coil assemblies, each ofwhich is, in the most frequent case, mounted on the outside of themeasuring tube along a diameter of the latter. Each of the coilassemblies commonly includes an air-core coil or a coil with a core ofsoft magnetic material.

To ensure that the magnetic field produced by the coils is ashomogeneous as possible, the coils are, in the most frequent andsimplest case, identical and electrically connected in series, thusaiding one another, so that in operation they can be traversed by thesame excitation current. It is also known, however, to pass anexcitation current through the coils alternatingly in the same directionand in opposite directions so as to be able to determine, for example,the viscosity of liquids and/or a degree of turbulence of the flow; see,in this connection, also EP-A 1 275 940, EP-A 770 855, or DE-A 43 26991.

The excitation current just mentioned is produced by an operatingelectronics; the current is regulated at a constant value of, e.g., 85mA, and its direction is periodically reversed. The current reversal isachieved by incorporating the coils in a so-called T network or aso-called H network; for the current regulation and current reversal,see U.S. Pat. No. 4,410,926 or U.S. Pat. No. 6,031,740.

The mentioned, induced voltage appears between at least two galvanic(thus, wetted by the liquid), measuring electrodes or between at leasttwo capacitive (thus, arranged within the wall of the measuring tube),measuring electrodes, with each of the electrodes picking up a separatepotential.

In the most frequent case, the electrodes are mounted at diametricallyopposed positions such that their common diameter is perpendicular tothe direction of the magnetic field, and thus perpendicular to thediameter on which the coil assemblies are located. The induced voltageis amplified, and the amplified voltage is conditioned by means of anevaluation circuit to obtain a measurement signal which is recorded,indicated, or further processed. Suitable evaluation electronics arefamiliar to those skilled in the art, for example from EP-A 814 324,EP-A 521 169, or WO-A 01/90702.

In principle, the absolute value of the potential at the respectiveelectrode is of no significance for the measurement of the volumetricflow rate, but only on condition that, on the one hand, the potentialslie in the dynamic range of a differential amplifier following theelectrodes, i.e., that this amplifier must not be overdriven by thepotentials, and that, on the other hand, the frequency of potentialchanges differs significantly from the frequency of the above-mentionedcurrent direction reversal.

The potential at each electrode is not only dependent on the magneticfield according to Faraday's law—the geometrical/spatial dimensions ofthe measuring tube and the properties of the liquid enter into thisdependence—, but this measurement signal, which is based on Faraday'slaw and should be as clean as possible, has interfering potentials ofdifferent geneses superimposed on it, as already discussed in EP-A 1 273892 or EP-A 1 273 891. These interfering potentials can contributesubstantially to a degradation of the measurement accuracy.

A first kind of interfering potential results from inductive and/orcapacitive interference which emanates from the coil assemblies andtheir leads, and which changes the electric charge on the capacitor thatexists at the boundary layer between electrode and liquid. As a resultof asymmetries in the concrete structure of the flow sensor,particularly as far as the conductor routing to the coil assemblies andto the measuring electrodes is concerned, the interfering potential ofone electrode generally differs from the interfering potential of theother electrode.

This—first—effect may, on the one hand, restrict the dynamics of thedifferential amplifier. On the other hand, the value of the differencebetween the interfering potentials of the electrodes is subject tovariances in flow-sensor parameters due to manufacturing tolerances.Also, the determinable dependence of the electrode potentials on thevelocity of the liquid is partly due to this effect, because at lowvelocities, the above-mentioned charges at the boundary layer betweenelectrode and liquid are not removed by the latter.

A second kind of interfering potential is caused by particles of foreignmatter or by air bubbles which are entrained by the liquid and which,when colliding with an electrode, cause sudden changes in the potentialof the electrode. The decay time of these changes is dependent on thetype of liquid and is generally greater than the rise time of thechanges.

This—second—effect, too, results in an erroneous measurement signal. Theerror is also dependent on the potential of the electrode. Since thispotential varies from flow sensor to flow sensor due to manufacturingtolerances, as was explained above, the second effect adds to the firsteffect, so that the individual flow sensor units differ widely in theirbehaviors, this being, of course, highly undesirable.

A third kind of interfering potential is caused by coatings deposited bythe liquid on the measuring electrodes, as is also described in U.S.Pat. No. 5,210,496, for example. The formation of the coatings is verystrongly dependent on the velocity of the liquid. The differences in thebehavior of the individual flow sensor units may be further increased bythe formation of the coatings.

EP-A 1 273 892 proposes a method of operating an electromagnetic flowsensor in which the development of the above-mentioned interferingpotentials of whatever kind is prevented, or at least their effect issignificantly reduced, by at least intermittently applying voltagepulses generated by means of the evaluation and operating circuit to atleast one of the two measuring electrodes. The use of this method canlead to a considerable improvement in the accuracy of electromagneticflowmeters, particularly in the case of single-phase or thoroughly mixedmultiphase liquids. Beyond this, in EP-A 337 292 or WO-A 03/004977, forexample, methods are described in which the measuring electrodes,particularly by being short-circuited to ground in timed sequence or byapplication of a harmonic alternating voltage, are subjected over aprolonged period of time to an interfering-potential-eliminatingdischarge voltage.

One disadvantage of this prior-art method of measurement, and of flowsensors using this method, is that in the case of multiphase liquidswith distinctly separated liquid phases or in the case of pasty-viscousliquids, for example, a rather stochastic, practically inestimabledistribution of the entrained particles of foreign matter or of the gasbubbles is to be expected, which can hardly be calibrated. To acorresponding extent, at least interfering potentials of the second kindcannot be sufficiently reliably removed from the measuring electrodes.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a method wherebythe effects of the aforementioned interfering potentials, particularlyof the potentials of the second kind, can be largely compensated for, sothat a measured value can be obtained which is substantially independentof such interfering potentials, particularly of potentials of the secondkind.

To attain this object, the invention provides a method of operating aprocess measuring device, especially an electromagnetic flowmeter havinga measuring tube inserted in a line conducting a medium, especially afluid medium, which method comprises the steps of:

-   -   causing the fluid to flow through the measuring tube;    -   causing an electrical, particularly bipolar, excitation current        to flow through an operating circuit of the flowmeter to drive        an excitation arrangement arranged on the measuring tube and        acting on the measuring tube and/or on the medium flowing        therethrough;    -   producing, by means of a sensor arrangement arranged on the        measuring tube, at least one, electrical, measurement signal        corresponding to the physical, measured variable;    -   digitizing the measurement signal or at least a portion thereof        to generate a digital sampling sequence representative of a        waveform of the measurement signal;    -   storing at least a part of the digital sampling sequence to        generate a first data set, which represents, instantaneously, a        waveform of the measurement signal within a predeterminable time        interval;    -   detecting an anomaly in the waveform of the measurement signal        caused at least in part by an, especially pulse-shaped,        interfering potential contained in the measurement signal, by        detecting within the stored first data set a data group which        digitally represents the anomaly;    -   extracting the data belonging to the data group from the stored        first data set to generate an interference-free second data set;        and    -   determining, using the second data set, a measured value        representative of a physical variable of the flowing fluid.

Furthermore, the invention provides an electromagnetic flowmeter for afluid flowing in a line, the flowmeter comprising:

-   -   a measuring tube, designed to be inserted into the line, for        conducting the fluid;    -   an evaluation and operating circuit;    -   means, fed by the evaluation and operating circuit, for        producing a magnetic field cutting the measuring tube, the means        comprising a coil arrangement mounted on the measuring tube and        traversed by an excitation current;    -   at least two measuring electrodes for picking up potentials        induced in the fluid flowing through the measuring tube and cut        by the magnetic field;    -   means, connected at least intermittently to the measuring        electrodes, for generating at least one measurement signal        derived from the potentials picked up by the measuring        electrodes; and    -   means for storing a first data set comprised of digitized        measurement data and representing, instantaneously, a waveform        of the measurement signal within a predeterminable time        interval;    -   wherein the evaluation and operating circuit    -   detects, by means of the first data set, an anomaly in the        measurement signal caused by an interfering potential appearing        at at least one of the measuring electrodes,    -   extracts the detected anomaly from the first stored data set and        generates a second data set free from the detected anomaly, and    -   generates by means of the anomaly-free data set at least one        measured value representative of a physical variable of the        flowing fluid.

In a first preferred embodiment of the method of the invention, thesecond data set also includes digital measurement data originallycontained in the first data set.

In a second preferred embodiment of the method of the invention, thestep of detecting the anomaly comprises the step of determining a firsttime value by means of the first data set, which time value representsan instant of the start of an interference voltage corresponding to theinterference potential.

In a third preferred embodiment of the method of the invention, the stepof determining the first time value comprises the steps of comparing thedigital data of the first data set with a predeterminable firstthreshold value and generating a first comparison value, which signalsan exceeding of the first threshold value.

In a fourth preferred embodiment of the method of the invention, thestep of detecting the anomaly comprises the step of determining a secondtime value by means of the first data set, which second time valuerepresents an instant of the ending of the interference voltage.

In a fifth preferred embodiment of the method of the invention, the stepof determining the second time value comprises the steps of comparingthe digital data of the first data set with a predeterminable secondthreshold value and generating a second comparison value, which signalsa subceeding of the second threshold value, i.e. the second thresholdvalue has been fallen below.

In a sixth preferred embodiment of the method of the invention, the stepof detecting the anomaly comprises the step of determining an amplitudevalue by means of the first data set, which amplitude value representsan amplitude, especially a maximum absolute amplitude, of themeasurement signal within the predeterminable time interval.

In a seventh preferred embodiment of the method of the invention, thestep of detecting the anomaly comprises the step of determining a thirdtime value by means of the first data set, which third time valuerepresents an instant of the occurrence of the amplitude, especially themaximum absolute amplitude, of the measurement signal within thepredeterminable time interval.

In an eighth preferred embodiment of the method of the invention, thestep of detecting the anomaly comprises the step of forming a timedifference between the first time value and the second time value todetermine a fourth time value representing the duration of theoccurrence of the interference voltage.

In a ninth preferred embodiment of the method of the invention, the stepof detecting the anomaly comprises the steps of comparing the amplitudevalue with a predeterminable third threshold value, particularly with athreshold value variable during operation, and generating a thirdcomparison value, which signals an exceeding of the third thresholdvalue.

In a tenth preferred embodiment of the method of the invention, the stepof generating the interference-free, second data set comprises the stepof determining, using the measurement signal, particularly the digitizedmeasurement signal, an average value for the voltage induced in theflowing fluid.

In an eleventh preferred embodiment of the method of the invention, thestep of generating the interference-free, second data set comprises thestep of determining, using digital data of the first data set, anaverage value for the voltage induced in the flowing fluid.

In a twelfth preferred embodiment of the method of the invention, thestep of generating the interference-free second data set comprises thestep of determining, using digital data having a time value less thanthe first time value, an average value for the voltage induced in theflowing fluid.

In a thirteenth preferred embodiment of the method of the invention, thestep of generating the interference-free second data set comprises thestep of determining an average value for the voltage induced in theflowing fluid using digital data with a time value greater than thesecond time value.

In a fourteenth preferred embodiment of the method of the invention, thestep of generating the interference-free second data set comprises thestep of generating an artificial third data set of digital data using atleast part of the data from the data group representative of theanomaly, which third data set approximates the waveform of theinterference voltage.

In a fifteenth preferred embodiment of the method of the invention, thestep of generating the artificial third data set comprises the step ofdetermining at least one regression, or data-fitting, function for atleast part of the digital data from the data group representative of theanomaly.

In a sixteenth preferred embodiment of the method of the invention, thestep of generating the artificial third data set comprises the step ofgenerating digital data using data values from the data grouprepresentative of the anomaly and using the determined regressionfunction.

In a seventeenth preferred embodiment of the method of the invention,the step of generating the interference-free second data set comprisesthe step of forming a difference between one of the data values from thedata group representative of the anomaly and one of the data values fromthe artificial third data set, the respective two data values used forforming the difference having corresponding, especially identical, timevalues.

In an eighteenth preferred embodiment of the method of the invention,the step of generating the at least one regression function comprisesthe step of determining, using data values from the data grouprepresentative of the anomaly, at least one coefficient, particularly atime constant, for the regression function.

In a nineteenth preferred embodiment of the method of the invention, thestep of generating the at least one regression function comprises thestep of determining a coefficient, particularly a time constant, for theregression function, using the average value determined instantaneouslyfor the voltage induced in the flowing fluid.

In a twentieth preferred embodiment of the method of the invention, thestep of determining the coefficient for the regression functioncomprises the steps of forming a first difference between a first datavalue from the data group representative of the anomaly and the averagevalue instantaneously determined for the voltage induced in the flowingfluid, forming a second difference between a second data value from thedata group representative of the anomaly and the average valueinstantaneously determined for the voltage induced in the flowing fluid,and forming a quotient of the first difference and the seconddifference.

In a twenty-first preferred embodiment of the method of the invention,the step of determining the coefficient for the regression functioncomprises the steps of generating a digital sequence of provisionalcoefficients for the regression function and digital, especiallyrecursive, filtering of the digital sequence of provisionalcoefficients.

In a twenty-second preferred embodiment of the method of the invention,the step of generating the third data set comprises the step ofdetermining at least a second regression function for at least a secondpart of the digital data from the data group representative of theanomaly.

According to a further development of the method of the invention, theexcitation arrangement which is used comprises an arrangement of coilsfor producing a magnetic field, especially also the magnetic fieldcutting through the medium conducted in the measuring tube.

According to a preferred embodiment of this further development of theinvention, the sensor arrangement which is used comprises measuringelectrodes arranged on the measuring tube and the method comprises thefollowing steps:

-   -   Producing by means of the excitation arrangement a magnetic        field also cutting through the fluid;    -   inducing a voltage in the flowing fluid for changing potentials        applied to the measuring electrodes; and    -   taking potentials applied to the measuring electrodes for        producing the at least one measurement signal.

A basic idea of the invention is to detect, on the basis of anomaliescorresponding to the interference potentials, the widely varyinginterfering potentials in at least one measurement signal, especiallydirectly and in the time domain or, rather, in the sampling time domain,on the basis of anomalies which occur in at least one measurement signaldelivered by the sensor arrangement of the flow sensor, or in digitallystored data sets derived from the measurement signal. By extracting thedigital data corresponding to the anomalies and replacing such bycalculated data, data sets are created, which are essentially composed,in part, of original measurement data and, in part, of synthetic,calculated data.

The invention is based on the surprising discovery that whileinterfering potentials of the kind described may be highlystochastically distributed, the anomalies to be detected generally havea typical characteristic or typical form whose detection makes itpossible both to identify such interfering potentials in the digitallystored data records derived from the measurement signal and to eliminatesuch interfering potentials by manipulation, particularly by nonlineardigital filtering, of the digital data affected by the interferingpotentials, with the information originally contained in the measurementsignal about the physical variable to be measured being, on the onehand, largely preserved and, being, on the other hand, made very rapidlyavailable for the determination of the measured value.

The method of the invention and further advantages will now be explainedin greater detail on the basis of waveforms and schematic circuitdiagrams presented in the figures of the drawing, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a, b show schematically and partly in block-diagram form aprocess measuring device—here in the form of an electromagneticflowmeter—suitable for carrying out the method of the invention;

FIG. 2 a shows a waveform of an excitation current flowing duringoperation of the process measuring device of FIGS. 1 a, 1 b;

FIGS. 2 b, c; show waveforms of potentials measurable during operationof the

FIGS. 3 a, b; process measuring device of FIGS. 1 a, 1 b; and

FIGS. 4 a, b;

FIGS. 5 a, b

FIGS. 6 a, b show digitally stored curves of potentials measured duringoperation of the process measuring device of FIGS. 1 a, 1 b.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the invention is susceptible to various modifications andalternative forms, exemplary embodiments thereof have been shown by wayof example in the drawings and will herein be described in detail. Itshould be understood, however, that there is no intent to limit theinvention to the particular forms disclosed, but on the contrary, theintention is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theappended claims.

FIG. 1 shows schematically and partly in the form of a block-diagram aprocess measuring device—in this instance an electromagneticflowmeter—suitable for carrying out the method of the invention. Theprocess measuring device is designed to produce measured values of leastone physical variable of a medium, especially a fluid, flowing in apipeline (not shown). For example, the flowmeter can be used to measurea volumetric flow rate and/or a flow velocity of an electricallyconductive liquid.

The flowmeter illustrated here includes a flow sensor 1 for generatingmeasurement potentials corresponding to the physical variable to bemeasured, an operating circuit 2 for collecting the measurementpotentials and generating at least one measurement signal correspondingto the physical variable, and an evaluation circuit 3 designed tocontrol the operating circuit 2, and thus the flow sensor 1, and togenerate measured values representative of the physical variable on thebasis of the at least one measurement signal. Operating circuit 2, andpossibly also some components of flow sensor 1, can, for example, behoused in an electronics case 10 of the flowmeter, as indicatedschematically in FIG. 1 a.

Flow sensor 1 includes a measuring tube 11, which is designed to beinserted in the aforementioned pipeline and has a tube wall and throughwhich during operation the fluid to be measured is made to flow in thedirection of a longitudinal axis of the tube.

To prevent a short circuit from being created for voltages induced inthe fluid, an inner portion of measuring tube 11, which contacts thefluid, is made electrically nonconductive. Metal measuring tubes are,for such purpose, commonly provided with an electrically nonconductivelining, e.g., a lining of hard rubber, polyfluoroethylene, etc., and aregenerally non-ferromagnetic; in the case of measuring tubes madeentirely of plastic or ceramic, particularly of alumina ceramic, theelectrically non-conductive lining is not necessary.

An excitation arrangement of the flowmeter, actuated by driverelectronics 21 provided in operating circuit 2, includes, in thisexample, a first field coil 12 and a second field coil 13, which arearranged on measuring tube 11. Field coils 12, 13 are located on a firstdiameter of the measuring tube 11. The excitation arrangement serves toproduce a magnetic field H which cuts the tube wall and the fluidflowing through the tube. The magnetic field is set up when anexcitation current I driven by driver electronics 21 is passed throughfield coils 12, 13, which in this embodiment are connected in series.The preferably bipolar excitation current I can be in the form of arectangular-wave, triangular, or sine-wave current, for example.

FIG. 1 b shows that field coils 12, 13 do not contain a core, i.e., thatthey are air coils.

However, as is usual with such coil arrangements, field coils 12, 13 mayalso be wound on a core which will generally be soft magnetic, and thecores may cooperate with pole pieces; see, for instance, U.S. Pat. No.5,540,103.

The excitation arrangement formed in the illustrated embodiment as acoil arrangement electromagnetically acting on the medium is herepreferably so designed, and the two field coils 12, 13, in particular,are so shaped and dimensioned, that within measuring tube 11, themagnetic field H produced with the two coils is symmetric, particularlyrotationally symmetric, with respect to a second diameter which isperpendicular to the first diameter.

In one embodiment of the invention, driver electronics 21 generates adirect current, especially a current regulated at a constant amplitude,which is then periodically switched by means of a suitable switchingcircuit, e.g., a circuit configured as a H or T network, and thusmodulated to obtain an alternating current of controlled amplitude.Thus, the excitation current I is made to flow through the coilarrangement in such a way that coils 12, 13, as shown schematically inFIG. 2 a, are each traversed in a first current direction during a firstswitching phase PH11 and in a direction opposite to the first directionduring a subsequent, second switching phase PH12; for the currentregulation and reversal, see also U.S. Pat. No. 4,410,926 or U.S. Pat.No. 6,031,740.

The second switching phase PH12 is followed by a third switching phasePH21, during which the excitation current I flows in the first directionagain. The third switching phase is followed by a fourth switching phasePH22, during which the excitation current I flows in the oppositedirection again. This is followed by a switching phase PH31, and soforth. With respect to the reversal of the direction of the excitationcurrent 1, every two successive switching phases form a switching periodP1, P2, P3, etc. Together with the reversal of the excitation current Iflowing through the coil arrangement, aside from a possibleswitching-phase shift essentially synchronous therewith, the polarity ofthe magnetic field H is repeatedly reversed, cf. FIG. 2 a.

For producing at least one electrical measurement signal correspondingto the measured variable, a sensor arrangement is provided in themeasurement sensor, arranged on the measuring tube or at least in itsvicinity. According to a preferred embodiment of the invention, thesensor arrangement includes electrodes mounted essentially directly tothe measuring tube. A first electrode 14 mounted on the inside of thewall of measuring tube 11 serves to pick up a first potential e₁₄induced by the magnetic field H. A second electrode 15 on the inside ofthe tube wall serves to pick up a second potential e₁₅ induced by themagnetic field. Electrodes 14, 15 are located on the measuring tubesecond diameter, which is perpendicular to the first diameter and to thelongitudinal axis of the measuring tube; they may also be located, forexample, on a chord of measuring tube 11 which is parallel to the seconddiameter, see also U.S. Pat. No. 5,646,353.

In FIG. 1 b, the measuring electrodes 14, 15 are galvanic electrodes,i.e., electrodes which contact the fluid. It is also possible to use twocapacitive electrodes, i.e., electrodes fitted in the wall of measuringtube 11, for example. Each of the electrodes 14, 15 picks up a separateelectric potential e₁₄, e₁₅, which in operation is induced, according toFaraday's law, in the fluid flowing through the measuring tube.

As shown in FIG. 1 b, in operation, electrodes 14 and 15 are at leastintermittently connected to a non-inverting input and an invertinginput, respectively, of a differential amplifier 22. Thus, a differenceof the two potentials e₁₄, e₁₅ picked up by electrodes 14, 15 is formed,which corresponds to a voltage developed in the flowing fluid, and thusto the physical variable to be measured, and which serves as ameasurement signal u. The potentials e₁₄, e₁₅ at electrodes 14, 15 aregenerally in a range of 10 to 100 mV.

As shown schematically in FIGS. 1 a and 1 b, the measurement signal uoccurring at the output of differential amplifier 22 is fed toevaluation circuit 3 provided in the flowmeter. According to theinvention, evaluation circuit 3 serves in particular to digitize thereceived measurement signal u and store it sectionally in the form of afirst data set DS₁, so that information about the waveform of a sectionof the measurement signal u is available in digital form for thedetermination of the measured value X_(M).

To this end, in evaluation circuit 3, the measurement signal u, as shownschematically in FIG. 1 a, is preferably first passed through a low-passfilter 31 of predeterminable order and adjustable cutoff frequency,e.g., a passive or active, RC filter. Low-pass filter 31 serves toband-limit the measurement signal u, in order to avoid aliasing, andthus preprocesses the signal for digitization. According to thewell-known Nyquist theorem, the cutoff frequency is set to less than 0.5times the rate at which the passed component of the measurement signal uis sampled. If the measurement signal u has already been band-limited inthe necessary manner, low-pass filter 31 can be dispensed with.

The output of low-pass filter 31 is coupled to a signal input of ananalog-to-digital (A/D) converter 32 of evaluation circuit 3, whichconverts the measurement signal u received from low-pass filter 31 to acorresponding, digital measurement signal u_(D). A/D converter 32 can beany of the A/D converters familiar to those skilled in the art, e.g.,converters using serial or parallel conversion, which can be clocked atthe above-mentioned sampling rate. A suitable A/D converter type is, forexample, the delta-sigma A/D converter ADS 1252 of Texas InstrumentsInc. with 24-bit resolution and a permissible sampling rate less than orequal to 40 kHz, it being understood that sampling rates less than 10kHz can be quite sufficient for the implementation of the methodaccording to the invention.

If A/D converter 32, e.g., the aforementioned ADS 1252, is provided forconverting exclusively positive signal values, a reference voltage ofthe converter must be chosen so that a minimum signal value to beexpected at the input of the converter will set at least one bit,particularly the most significant bit (MSB), of the measurement signalu_(D). In other words, a DC component must be added to the output signalof low-pass filter 31 so that this signal will act on A/D converter 32essentially as a DC signal of variable amplitude.

The measurement signal u_(D) provided at the output of A/D converter 32is loaded segment by segment, e.g. via an internal data bus, into avolatile data memory 33 of evaluation circuit 3, where it is keptavailable as a finite sampling sequence AF instantaneously representingthe measurement signal u in the form of an ensemble of digitally storedmeasurement data, particularly for a digital flow-computer 34 ofevaluation circuit 3. Data memory 33 can be implemented with a static ordynamic, random-access memory, for example.

A width for an instantaneous sampling window, i.e., a time length of thesection of the sampling sequence AF to be stored, for instantaneouslyrepresenting the measurement signal u, may lie, for example, in therange of the total duration of one of the switching periods P1, P2 withwhich the excitation current I is clocked, or in the range of theduration of one of the switching phases PH11, PH12, PH21, PH22.Accordingly, the clocking for reading into data memory 33 is essentiallyin phase with the clocking of the excitation current. Clock periodscommonly used in conventional flowmeters of the kind described areapproximately in the range of 10 to 100 ms; at a 10-kHz sampling ratef_(a) of A/D converter 32, this would give 100 to 1000 samples of thesampling sequence AF, or the first data set.

If necessary, e.g. because of a lower capacity of data memory 33 or inorder to eliminate introductory voltage transients caused by fieldreversals, it is also possible to read into data memory 33 only aportion of the measurement signal u, or, rather, the digital measurementsignal u_(D), generated per switching phase. To illustrate this, each ofthe above-mentioned switching phases PH11, PH12, PH21, PH22, PH31 issubdivided into a first subperiod T111, T121, T211, T221, T311, servingto establish the magnetic field, and an associated second subperiodT112, T122, T212, T222, T312, serving as a measurement phase; cf. FIGS.2 a, 2 b, and 2 c. Preferably, in this embodiment of the invention, onlya waveform of the measurement signal u associated with the respectivesecond subperiod T112, T122, T212, T222, T312 is virtually mapped indata memory 33, with the evaluation of the measurement data and thegeneration of the measured value taking place during the respective nextmagnetic field establishment phase T121, T211, T221, T311.

To generate the measured value X_(M) from the sampling sequence AF, flowcomputer 34 has an at least temporary access, particularly a datareading access, to data memory 33 and the data sets stored therein, e.g.via the internal data bus. Flow computer 34 is advantageouslyimplemented with a microprocessor 30 and computing programs runningtherein, as shown schematically in FIG. 1 a.

In a preferred embodiment of the invention, evaluation circuit 3 furthercomprises a memory manager 35 implemented as a separate subcircuitwhich, communicating with microprocessor 30, e.g. via the internal databus, serves to manage data memory 33, especially to control the samplingof the digital measurement signal u_(D) and the generation of thesampling sequence AF, thus reducing the load on microprocessor 30.Memory manager 35 is preferably implemented with a programmable logicdevice, such as a PAL (programmable array logic) or an FPGA(field-programmable gate array). If necessary, memory manager 35 canalso be implemented with microprocessor 30 or with a furthermicroprocessor (not shown) and suitable computing programs runningtherein. By means of memory manager 35 it is also possible to implement,for example, the averaging or the determination of the median usual forsuch flowmeters, this to be done over plural sampling sequences.

As mentioned, due to interfering potentials E112, E122, E222, E312appearing at the measuring electrodes 14, 15, the measurement signal umay be severely disturbed and thus corrupted; see also FIGS. 2 b, 2 c.To illustrate this, FIGS. 3 a, 3 b show waveforms of the potentials e₁₄,e₁₅ recorded over approximately ten seconds, on which interferingpotentials are superimposed. In FIGS. 4 a, 4 b, portions of the recordedpotential waveforms e₁₄, e₁₅ which are disturbed in the manner describedare shown again on another time scale; virtually interference-freeportions of the potential waveforms e₁₄, e₁₅ shown in FIGS. 3 a, 3 b areagain illustrated in FIGS. 5 a, 5 b.

Investigations of the waveforms of such interfering potentials haveshown that, while the amplitudes of such interfering potentials or theinstants of their occurrence, for example, are not predeterminable, atypical amplitude characteristic can be postulated at least for a greatnumber of interfering potentials and taken into account as a-prioriinformation in the evaluation of the measurement signal u and in thedetermination of the measured values. To the inventors' surprise, itturned out that the interfering potentials are reflected in the waveformof the measurement signal u in the form of distinct anomalies, whosecharacteristic can be determined beforehand, at least qualitatively. Itturned out that, in operation, these anomalies can be comparativelyreliably detected within the sampling sequence or, rather, by means ofthe current data set DS₁ derived therefrom, and can be eliminated fromthe data set at the expense of a comparatively very small loss ofinformation.

Accordingly, in the method according to the invention, an anomaly in thewaveform of the measurement signal u caused at least in part by aninterfering potential, particularly by a pulse-shaped interferencevoltage, appearing at at least one of the measuring electrodes 14, 15 isdetected, as schematically illustrated in FIG. 6 a, by determiningwithin the stored first data set DS₁ a data group DS_(A) whichrepresents the anomaly in digital form. Furthermore, in order togenerate an interference-free second data set DS₂, the anomaly thusdetected is extracted from the stored first data set DS₁, with theresulting interference-free data set DS₂ then being used to determinethe measured value X_(M), which represents the physical variable to bemeasured for the flowing fluid.

In one embodiment of the method of the invention, in order to generatethe interference-free data set DS₂, an average value U of the voltageinduced in the flowing fluid is determined using a portion of themeasurement signal u, or of the already digitized measured signal u_(D),and kept available in data memory 33 for further computations.Advantageously, the average value U can be determined using thecurrently stored data set DS₁ and/or a data set which was derived fromthe measurement signal u during an earlier switching phase, preferablyduring the immediately preceding switching phase or during the precedingswitching phase of the same current direction, and temporarily stored.Preferably, data, which does not belong to the data group DS_(A)representing the anomaly and can thus be regarded as essentiallyinterference-free, is used for the determination of the average value U.

Thus, using the average value U, interference can be very effectivelyeliminated from the data set DS₁ in a simple manner by simply erasingthe individual data of the data group DS_(A) representing the anomalyfrom the currently stored data set DS₁ and putting the respectiveinstantaneous average value U in the “vacated” places of the data setDS₁. During wide variations of the flow rate, however, a considerableamount of measurement information may be lost in this manner.

Based on the recognition that most interfering potentials have,qualitatively, essentially comparable waveforms, which are thus at leastqualitatively determinable beforehand or at least readily estimable,according to a further development of the method of the invention, theinterference-free second data set DS₂, as indicated in FIGS. 6 b, isformed using a data set DS_(K) of artificially generated digital data(third data set) which approximates the waveform of the interferencevoltage. This data is computed by evaluation circuit 3 using at leastpart of the data from the previously located data group DS_(A)representing the anomaly and temporarily stored in data memory 33 ifnecessary.

The interference-free second data set DS₂ can now advantageously begenerated by first selecting a respective data value x from the datagroup DS_(A), which represents the anomaly, and from the third data setDS_(K), with the two selected data values x having corresponding,especially identical, time values i, and by forming a numericaldifference of the two selected data values x. This is repeated until allthe data values x from the data group DS_(A) have been used. In thismanner, an interfering potential approximated in its waveform,particularly in its amplitude and duration, is subtracted from themeasurement signal u in a virtual mode. Thus, assuming that the voltageexceeding the approximated waveform of the interfering potential isessentially the actual measurement voltage of interest, only the portionof the measurement signal corresponding to the physical variable remainsin the interference-free data set DS₂.

To generate the artificial data set DS_(K), according to a furtherdevelopment of the invention, evaluation circuit 3 determines at leastone regression function for at least part of the digital data from thedata group DS_(A) representing the anomaly and uses this regressionfunction to determine the artificial data set DS_(K). In one embodimentof this development of the method of the invention, at least onecoefficient T₁, but preferably two or more, are determined for the atleast one regression function using data values x from the data groupDS_(A).

To determine the regression function, especially the coefficients T₁ forthe regression function, an algorithm based on Gauss' principle of leastsquares, for example, can be programmed into evaluation circuit 3 andapplied to the data group DS_(A) currently available in data memory 33.

Compared with the above embodiment in which only the current averagevalue U is used as a substitute for the data from the data group DS_(A)representing the anomaly, the use of a suitable regression function,particularly the use of the interference-free data set DS₂ thusgenerated, made it possible to at least halve, and thus significantlyfurther reduce, the measurement error.

Investigations of representative applications have also shown that aparticularly frequently occurring waveform of interfering potentials ofthe kind described is very similar to that of short-duration,needle-shaped voltage pulses, for example. The interfering potentialshave a generally relatively steeply rising edge followed by anessentially exponentially falling edge. Based on this recognition, inanother embodiment of the aforementioned development of the invention,at least one coefficient for the at least one regression function isdetermined as a time constant of an exponentially decreasing, e.g.,first- or higher-order, regression function.

In a further embodiment of the invention, provisional coefficients,particularly a sequence of provisional coefficients, are first generatedfor the regression function, e.g. by repeated sequential application ofthe aforementioned computation rule to different data pairs from thedata group DS_(A) representing the anomaly. According to a furtherdevelopment, the provisional coefficients determined are digitallyfiltered, for instance individually immediately after their computationor only after the computation of the entire sequence of coefficients.Investigations have shown that, particularly if a recursive digitalfilter is used for the sequence of coefficients, good measurementresults, particularly robust results and results which are accuratelyreproducible even in the presence of a broad spectrum in the interferingpotentials, can be achieved even with a low-order filter. In a preferredembodiment, the sequence of provisional coefficients can be determinedaccording to the following formation rule,{acute over (T)} _(n) =λ·T _(n)+(1−λ)·{acute over (T)} _(n−1),  (1)where

-   T_(n)—a provisional coefficient for the regression function,    computed in the currently executed, computation step,-   T_(n−1)—a provisional coefficient for the regression function,    computed in the preceding computation step,-   T_(n)—an intermediate value predetermined for the current    computation step, and-   λ,(1−λ) predetermined filter coefficients for the digital filter,    with 0<λ<1.

It is possible either to store the provisionally determined coefficientsindividually or to hold only the respective current and precedingcoefficients in data memory 33. The computation rule is applied until apredetermined number of loops, e.g., a number equal to the number ofdata in the data group DS_(A) representing the anomaly, has beenexecuted and/or until a previously selected abort criterion, e.g., asufficiently small change between last provisional coefficientscomputed, is satisfied. The last coefficient computed will thencorrespond to the coefficient T₁ sought for the regression function.

If an interfering potential appears between the measuring electrodesover a prolonged period of time, thus over several measurement phases,the coefficient will be computed using a corresponding coefficientdetermined in an immediately preceding measurement phase; this oldercoefficient can then serve as a current provisional coefficient T_(n−1),for example.

In a further embodiment of the method of the invention, the coefficientor coefficients for the at least one regression function are computedusing the instantaneous average value U determined for the voltageinduced in the flowing fluid. This can advantageously be implementednumerically already during the determination of the intermediate valuesfor the provisional coefficients based on the following computationrule:

$\begin{matrix}{T_{n} = \left. \frac{\left( {i_{1} - i_{2}} \right)}{\ln\left( \frac{x_{i2} - U}{x_{i1} - U} \right)} \right|_{n}} & (2)\end{matrix}$where

-   x_(i1), x_(i2)—are a first and a second data value from the data    group DS_(A) representing the anomaly, and-   i₁, i₂—are their subscripts, which correspond to the respective    associated time values.

In this embodiment of the method using Eq. (2), a first difference isformed between a first data value x_(i1) from the data group DS_(A)representing the anomaly and the instantaneous average value Udetermined for the voltage induced in the flowing fluid, and a seconddifference is formed between a second data value x_(i2) from the datagroup DS_(A) and the instantaneous average value U determined for thevoltage induced in the flowing fluid. For a quotient determined from thefirst and the second differences, the natural logarithm is determinednumerically, on which a previously formed difference between the timevalues or subscripts i₁, i₂ of the currently used data values x_(i1),x_(i2) is then normalized.

To detect the anomaly, in a further embodiment of the invention, a firsttime value t_(s), which represents an instant of the start of theinterference voltage, is determined by means of the first data set DS₁.To this end, the digital data of the first data set DS₁ can, forinstance, be compared with a predeterminable first threshold valueTH_(s), particularly a threshold value variable in operation, and afirst comparison value generated for signalling that the first thresholdvalue TH_(s) has been exceeded. The first time value t_(s) can becalculated from t_(s)=i_(s)/f_(a) where is i_(s) the subscript of thefirst data value determined to exceed the threshold value. Furthermore,to detect the anomaly by means of the first data set DS₁, a second timevalue t_(e) is determined, for example based on t_(e)=i_(e)/f_(a), whichrepresents an instant of the ending of the interference voltage. Inanalagous fashion, the digital data of the first data set DS₁ can becompared, for example, with a predeterminable second threshold valueTH_(e), particularly a threshold value variable in operation, togenerate a corresponding second comparison value signalling a subceedingof, or a falling beneath, the second threshold value TH_(e). At thispoint it should be noted that the aforementioned comparisons actuallyrelate to the absolute value of the measurement signal u. If thesecomparisons are to take into account the sign of the measurement signalu, the thresholds TH_(s), TH_(e) for negative voltages have to be fixedat the corresponding opposite values.

Based on the assumption that for physical or technological reasons, theflow rate between two successive measured phases T112 and T122, forexample, can only vary to a comparatively small extent, in a preferredembodiment of the invention, at least one of the threshold valuesTH_(s), TH_(e) is determined in operation and adapted to the fluidcurrently flowing in measuring tube 11, particularly to a flow ratevalue determined for an earlier switching phase. Advantageously, thethreshold value TH_(s) or TH_(e) can be formed using an average value Uof the measurement signal u determined in an earlier measurement phase,particularly in an immediately preceding or a youngest, undisturbedmeasurement phase, for example by a simply increasing the thresholdvalue during operation by a value corresponding to the maximum increaseof the measurement signal u to be expected within the meanwhile elapsedtime, or by a corresponding percentage.

In a further embodiment of the method of the invention, the anomaly isdetected by determining at least one amplitude value and an associatedthird time value by means of the first data set DS₁, with the amplitudevalue representing an amplitude, particularly a maximum absoluteamplitude, of the measurement signal within a predeterminable timeinterval. Furthermore, it is provided for detecting the anomaly thatseveral or all data of the first data set DS₁, or only the data for theamplitude value, are compared with a predeterminable third thresholdvalue TH_(a), particularly a threshold value variable in operation. Thisthreshold value is chosen to be greater than the first threshold valueTH_(s) and represents a predetermined minimum amplitude for a voltagerise to be detected as an anomaly. In addition to this, a correspondingthird comparison value is generated which signals that the thresholdvalue TH_(a) has been exceeded.

In another embodiment of the invention, the anomaly is detected bycomparing the digital data of the first data set DS₁ with apredeterminable third threshold value TH_(a) and generating the thirdcomparison value in a corresponding manner, which signals that thethreshold value TH_(a) has been exceeded.

In a further embodiment of the invention, in order to detect theanomaly, a time difference t_(e)−t_(s) is formed between the previouslydetermined first time value t_(s), representing the start of theinterference voltage, and the second time value t_(e), representing theend of the interference voltage, to determine a fourth time value, whichrepresents the duration of the occurrence of the interference voltage.This fourth time value can then be compared with a corresponding fourththreshold value, which represents a predeterminable minimum duration fora voltage pulse regarded as an anomaly to be eliminated.

Furthermore, in the case of a data set DS₁ which is not totallydisturbed, the average value U of the voltage induced in the flowingfluid can be calculated using digital data with a time value less thanthe previously determined first time value t_(s) and/or using digitaldata of the data set DS₁ having a time value greater than the secondtime value t_(e).

It is also possible to determine, in addition to the above-mentionedregression function, a further regression function for at least a secondpart of the digital data from the data group DS_(A) representing theanomaly, e.g., an ascending straight line for the leading edge of theinterference voltage pulse, and to generate the data of the artificialdata set DS_(K) also by using this second regression function.

Following the generation of the interference-free data set DS₂, thevalue representing the physical variable to be measured can becalculated by means of evaluation circuit 3 in the usual manner, forexample in the manner described in U.S. Pat. Nos. 4,382,387, 4,422,337,or U.S. Pat. No. 4,704,908 for a flow-rate measurement. Thedetermination of, e.g., the flow rate, as mentioned, is based on theevaluation of the instantaneous flowrate-dependent amplitudecharacteristic of the voltage taken between the two measuring electrodes14, 15, which can now be determined with a high degree of accuracy inthe conventional manner by means of the interference-free data set DS₂held in data memory 33. The current interference-free data set DS₂, orseveral such stored data sets, can also be used to determine furtherphysical quantities of interest, such as a viscosity of the fluid, aflow index, a degree of turbulence, or the like.

At this point it should be noted that instead of using a singledifferential amplifier for the measuring electrodes 14, 15 to generatean analog difference signal, a corresponding separate signal amplifiermay, of course, be provided for each of the electrodes 14, 15.Accordingly, the difference of the two potentials e₁₄, e₁₅ obtained fromelectrodes 14, 15 may also be calculated by means of two digitizedmeasurement signals and numerically.

Both the methods of evaluation required for generating theinterference-free data set DS₂ using the data set DS₁ and fordetermining the measured values X_(M) by means of the interference-freedata set DS₂ can be implemented in the manner familiar to those skilledin the art, e.g. as a computer program running in microprocessor 30. Thenecessary program codes can be readily implemented in a writable memory36 of evaluation stage 3, particularly in a permanent memory, such as anEPROM, an EEPROM, or a flash EEPROM, to which microprocessor 30 has readaccess during operation.

In a preferred embodiment, microprocessor 30 is implemented with adigital signal processor, e.g., of the type TMS 320 C 33 of TexasInstruments, Inc. In control unit 3, a signal processor, for example,may be provided in addition to microprocessor 30, if necessary.

The flowmeter may be connected to a fieldbus (not shown), for example,and thus linked to a remote control room and to an external power supplywhich feeds the flowmeter via an internal supply unit 4. To send meterdata, particularly the measured flow-rate value, to the fieldbus, theflowmeter further comprises a communications unit 5 with suitable datainterfaces 51. Furthermore, communications unit 5 may comprise a displayand control unit 52, particularly to visualize meter data and/or topermit on-site adjustment of the flowmeter.

Among the advantages of the invention is in the fact that, particularlysince it dispenses with higher-order digital filters for the samplingsequence AF or the first data set or with a complex spectral analysis ofthe same in the frequency domain, the measured value is determined aftera comparatively short time, even if the data set used is disturbed inwhole or in part. This can even be achieved for disturbances of themeasurement signal u which are present over two or more measurementphases. In addition, the method according to the invention, apart fromrequiring very little computing time, provides significantly higherselectivity with respect to disturbances of the kind described thandigital filters of correspondingly higher order which are comparablewith regard to their action. In particular, very good results can beachieved for interfering potentials of the above-mentioned second kindor in the case of high-viscosity liquids, such as pulp. A furtheradvantage of the invention is that the method can be implemented usingboth conventional flow sensors and conventional operating circuits. Evenconventional evaluation circuits can be used, if the implementedsoftware is modified in a suitable manner. Another advantage of themethod of the invention is that it can be used also for processmeasuring devices other than that of the example of an embodimentpresented here. For example, the method is very advantageously usablefor flowmeters working by means of ultrasonic sensors or by means ofmeasuring tubes that vibrate during operation.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and description isto be considered as exemplary and not restrictive in character, it beingunderstood that only exemplary embodiments have been shown and describedand that all changes and modifications that come within the spirit andscope of the invention as described herein are desired to protected.

1. A method of operating a process measuring device, especially anelectromagnetic flowmeter having a measuring tube inserted in a lineconducting a medium, especially a fluid medium, which method comprisesthe steps of: causing the fluid to flow through the measuring tube;causing an electrical, particularly bipolar, excitation current to flowthrough an operating circuit of the flowmeter to drive an excitationarrangement arranged on the measuring tube, and acting on the measuringtube and/or on the medium flowing therethrough; producing, by means of asensor arrangement arranged on the measuring tube, at least one,electrical, measurement signal (u) corresponding to the physical,measured variable; digitizing the measurement signal (u) or at least aportion thereof to generate a digital sampling sequence (AF)representative of a waveform of the measurement signal (u); storing atleast a part of the digital sampling sequence (AF) to generate a firstdata set (DS₁), which represents, instantaneously, a waveform of themeasurement signal (u) within a predeterminable time interval; detectingan anomaly in the waveform of the measurement signal caused at least inpart by an, especially pulse-shaped, interference potential (E222)contained in the measurement signal, by detecting within the storedfirst data set (DS₁) a data group (DS_(A)) which digitally representsthe anomaly; extracting the data belonging to the data group (DS_(A))from the stored first data set to generate an interference-free seconddata set (DS₂); and determining, using the second data set (DS₂), ameasured value (XM) representative of a physical variable of the flowingfluid.
 2. The method as claimed in claim 1, wherein: the second data set(DS₂) also includes digital measurement data originally contained in thefirst data set (DS₁).
 3. The method as claimed in claim 1, wherein: thestep of detecting the anomaly includes the step of detecting a firsttime value (t_(s)) on the basis of the first data set (DS₁), which timevalue (t_(s)) represents an instant of the start of an interferencevoltage corresponding to the interference potential (E222).
 4. Themethod as claimed in claim 1, wherein: the step of determining the firsttime value (t_(s)) comprises the steps of comparing the digital data ofthe first data set (DS₁) with a predeterminable first threshold value(TH_(s)) and generating a first comparison value, which signals that thefirst threshold value (TH_(s)) has been exceeded.
 5. The method asclaimed in claim 1, wherein: the step of detecting the anomaly comprisesthe step of determining a second time value (t_(e)) by means of thefirst data set (DS₁), which time value (t_(e)) represents an instant ofthe ending of the interference voltage.
 6. The method as claimed inclaim 1, wherein: the step of determining the second time value (t_(e))comprises the steps of comparing the digital data of the first data set(DS₁) with a predeterminable second threshold value (TH_(e)) andgenerating a second comparison value, which signals the subceeding ofthe second threshold value (TH_(e)).
 7. The method as claimed in claim1, wherein: the step of detecting the anomaly comprises the step ofdetermining an amplitude value by means of the first data set (DS₁),which amplitude value represents an amplitude, particularly a maximumabsolute amplitude, of the measurement signal (u) within apredeterminable time interval.
 8. The method as claimed in claim 1,wherein: the step of detecting the anomaly comprises the step ofdetermining a third time value by means of the first data set, whichtime value represents an instant of the occurrence of the amplitude,particularly the maximum absolute amplitude, of the measurement signalwithin the predeterminable time interval.
 9. The method as claimed inclaim 6, wherein: the step of detecting the anomaly comprises the stepsof comparing the amplitude value with a predeterminable third thresholdvalue (TH_(a)), particularly a threshold value variable in operation,and generating a third comparison value, which signals that the thirdthreshold value (TH_(a)) has been exceeded.
 10. The method as claimed inclaim 2, wherein: the step of detecting the anomaly comprises the stepof forming a time difference (t_(e)−t_(s)) between the first time value(t_(s)) and the second time value (t_(e)) to determine a fourth timevalue, which represents the duration of the occurrence of theinterference voltage.
 11. The method as claimed in claim 1, wherein: thestep of generating the interference-free second data set (DS₂) comprisesthe step of determining a average value (U) for the voltage induced inthe flowing fluid using the measurement signal (u), particularly thealready digitized measurement signal.
 12. The method as claimed in claim1, wherein: the step of generating the interference-free second data set(DS₂) comprises the step of determining an average value (U) for thevoltage induced in the flowing fluid using digital data of the firstdata set (DS₁).
 13. The method as claimed in claim 1, wherein: the stepof generating the interference-free, second data set comprises the stepof generating an artificial third data set (DS_(K)) using at least partof the data from the data group (DS_(A)) representative of the anomaly,which third data set (DS_(K)) approximates the waveform of theinterference voltage.
 14. The method as claimed in claim 1, wherein: thestep of generating the artificial third data set (DS_(K)) comprises thestep of determining at least one regression function for at least partof the digital data from the data group (DS_(A)) representative of theanomaly.
 15. The method as claimed in claim 12, wherein: the step ofgenerating the artificial data set (DS_(K)) comprises the step ofgenerating digital data using data values from the data group (DS_(A))representative of the anomaly and using the determined regressionfunction.
 16. The method as claimed in claim 12, wherein: the step ofgenerating the second data set (DS₂) comprises the step of forming adifference between one of the data values from the data group (DS_(A))representative of the anomaly and one of the data values from theartificial third data set (DS_(K)), the respective two data values usedfor forming the difference having corresponding, especially identical,time values.
 17. The method as claimed in claim 13, wherein: the step ofgenerating the at least one regression function comprises the step ofdetermining at least one coefficient (T₁), particularly a time constant,for the regression function using data values from the data group(DS_(A)) representative of the anomaly.
 18. The method as claimed inclaim 13, wherein: the step of generating the at least one regressionfunction comprises the step of determining a coefficient (T₁),particularly a time constant, for the regression function using theinstantaneous average value (U) determined for the voltage induced inthe flowing fluid.
 19. The method as claimed in claim 17, wherein: thestep of determining the coefficient (T₁) for the regression functioncomprises the steps of: forming a first difference between a first datavalue from the data group (DS_(A)) representative of the anomaly and theinstantaneous average value (U) determined for the voltage induced inthe flowing fluid; forming a second difference between a second datavalue from the data group (DS_(A)) representative of the anomaly and theinstantaneous average value (U) determined for the voltage induced inthe flowing fluid; and forming a quotient of the first difference andthe second difference.
 20. The method as claimed in claim 17 wherein:the step of determining the coefficient for the regression functioncomprises the steps of: generating a digital sequence (T_(n)) ofprovisional coefficients for the regression function; and digital,especially recursive, filtering of the digital sequence (T_(n)) ofprovisional coefficients.
 21. The method as claimed in claim 17,wherein: the step of generating the third data set (DS_(K)) comprisesthe step of determining at least a second regression function for atleast a second part of the digital data from the data group (DS_(A))representative of the anomaly.
 22. The method as claimed in claim 1,wherein: the excitation arrangement comprises a coil arrangement forgenerating a magnetic field (H), especially a magnetic field alsocutting through the medium conducted in the measuring tube.
 23. Themethod as claimed in claim 1, wherein: the sensor arrangement comprisesmeasuring electrodes arranged on the measuring tube, comprising thefollowing further steps: generating by means of the excitationarrangement a magnetic field (H) also cutting through the mediumconducted in the measuring tube; inducing a voltage in the flowing fluidfor changing potentials (e₁₄, e₁₅) applied to measuring electrodes; andtaking potentials (e₁₄, e₁₅) existing at the measuring electrodes forproducing the at least one measurement signal (u).
 24. Anelectromagnetic flowmeter for a fluid flowing in a line, comprising: ameasuring tube insertable into the line for conducting the fluid; anevaluation and operating circuit; means, fed by the evaluation andoperating circuit, for producing a magnetic field cutting the measuringtube, the means comprising a coil arrangement arranged on the measuringtube and traversed by an excitation current; at least two measuringelectrodes for picking up potentials (e₁₄, e₁₅) induced in the fluidflowing through the measuring tube and cut by the magnetic field; means,connected at least intermittently to said measuring electrodes, forgenerating at least one measurement signal (u) derived from thepotentials (e₁₄, e₁₅) picked up by said measuring electrodes; and meansfor storing a first data set (DS₁), which instantly represents awaveform of the measurement signal (u) within a predeterminable timeinterval; wherein: said evaluation and operating circuit detects; bymeans of said first data set (DS₁) an anomaly in the measurement signal(u) caused by an interfering potential appearing at at least one of saidmeasuring electrodes; extracts the detected anomaly from the storedfirst data set (DS₁) and generates a second data set (DS₂), which isfree from the detected anomaly; and generates by means of theanomaly-free data set (DS₂) at least one measured value (X_(M))representative of a physical variable of the flowing fluid.